Operation of internal combustion engines typically results in the generation of particulate matter, including inorganic species (ash), sulfates, small organic species generally referred to as soluble organic fraction (SOF), and hydrocarbon particulates or “soot.” Various strategies have been used for preventing the release of such particulate matter into the environment, including the use of particulate filters, available in a variety of designs. Particulate filters are used in both on-highway and off-highway applications, and typically include a porous ceramic material positioned in the path of exhaust exiting the engine. The particulate filter traps particulate matter in the exhaust gases, reducing its release via the engine tailpipe or exhaust stack.
Over time, accumulating particulate matter in a conventional filter will tend to impede exhaust flow through the filter, resulting in a pressure drop. Periodically, conventional filters need to be cleaned. A variety of strategies are known for “regenerating” a particulate filter, often by heating the particulate matter to a temperature at which the trapped particles combust or oxidize. Specialized hardware such as electrical heaters and fuel injection devices and/or engine operating strategies are commonly used for filter regeneration, adding expense, size and complexity to an engine system and its controls. Filters are also typically sized to allow a relatively large amount of particulate matter to be accumulated without substantially affecting performance, so that regeneration cycles are necessary relatively infrequently. In addition, filter volumes typically account for the need to trap ash from combusted particulates. Certain designs use precious metal catalysts to enable essentially continuous regeneration, reducing the need for other regeneration hardware or controls. These designs, however, have their own drawbacks, notably expense.
Further considerations in particulate filter and overall engine system design relate to the positioning of particulate filters within an engine system. In many modern internal combustion engine systems, particularly compression ignition diesel engine systems, one or more turbochargers are used to extract energy from engine exhaust. Conventional wisdom has been to position turbochargers upstream a particulate filter, in part due to the relatively higher energy of the exhaust prior to its passing through a particulate filter. A further consideration driving filter positioning has been the relative sensitivity of common filter materials to temperature conditions at different locations in an exhaust system. Exhaust temperatures upstream a turbocharger tend to vary relatively widely and rapidly, causing problems in conventional filter materials. The heat release associated with filter regeneration can further compound this problem. It will thus be recognized that many conventional particulate filters tend to be relatively large, complex and expensive pieces of equipment, with limited design and placement flexibility in an engine system.